This invention relates generally to turbofan gas turbine engines and more particularly to fan cases for such engines.
A turbofan gas turbine engine used for powering an aircraft in flight typically includes, in serial flow communication, a fan, a low pressure compressor or booster, a high pressure compressor, a combustor, a high pressure turbine, and a low pressure turbine. The combustor generates combustion gases that are channeled in succession to. the high pressure turbine where they are expanded to drive the high pressure turbine, and then to the low pressure turbine where they are further expanded to drive the low pressure turbine. The high pressure turbine is drivingly connected to the high pressure compressor via a first rotor shaft, and the low pressure turbine is drivingly connected to both the fan and the booster via a second rotor shaft.
The fan includes a plurality of circumferentially spaced apart fan blades extending radially outwardly from a rotor disk that is drivingly connected to the low pressure shaft. Each fan blade generally has an airfoil section and an integral dovetail root section that attaches the blade to the rotor disk. The fan is rotatively supported on a nonrotatable frame, commonly referred to as the fan frame, by a support system that typically includes a number of bearings and bearing support structure.
During engine operation, there is a remote possibility that a foreign body, such as a bird, could impact the fan and cause a fan blade-out event; i.e., part or all of a fan blade becomes detached from the rotor disk. Such a detached fan blade could cause considerable damage to the aircraft powered by the engine if it were not contained by the fan case. Various containment systems have been developed to prevent such damage. Fan blade containment systems have traditionally included an annular containment case manufactured from a high strength material with an adequate shell thickness to absorb the kinetic energy of an impacting fan blade. More recent containment systems have employed nesting areas defined by inner and outer annular shells having honeycomb structures disposed therein. In addition, ballistic material, such as an aromatic polyamide fiber, may be wrapped around the case structure. Blade fragments are captured in the nesting area and are thus contained within the system and prevented from further contact with other fan blades.
A fan blade-out event creates a large imbalance in the fan rotor, which could result in the transmission of potentially damaging imbalance forces to the fan frame. This is particularly the case with the increased fan size of recent commercial engine models, because the imbalance loads to be sustained during a fan blade-out event are increasingly large. One solution is to incorporate a decoupler in the fan rotor support structure. A decoupler is a frangible structure designed to fail in response to a fan blade-out load and allow the fan rotor to orbit about its new center of mass. Thus, the large imbalance loads are not transmitted to the fan frame. Accordingly, use of a decoupler effectively reduces the overall weight of the engine because the fan frame and related structure need not be made sufficiently strong to withstand substantial imbalance forces.
A fan case for a turbofan engine having a decoupler has two unique requirements. First, it must maintain a tight running clearance with the fan blade tips during normal engine operation while allowing the fan rotor to orbit after a fan blade-out event causes the decoupler to fail. A second requirement is to control the trajectory of secondary blade fragments. During a fan blade-out event, the released blade, and typically a portion of the first trailing blade broken off by the released blade, will break up upon impacting the fan case. It is desirable that these fragments exit the axial plane defined by the fan rotor so as to avoid additional secondary blade damage. Additional blade damage will increase the imbalance and aggravate the containment problem. Furthermore, it is desirable that the blade fragments exit the fan plane in the aft direction. This way, the fragments will impact the fan outlet guide vanes downstream of the fan. This secondary impact will either be sufficient to absorb the remaining fragment energy, or will fracture the outlet guide vanes and allow the fragments to escape through the fan flowpath aft of the engine, in the normal direction of the engine exhaust. Either outcome is acceptable in response to a fan blade-out event. However, if a blade fragment is deflected forward of the fan plane, the aircraft can be at hazard, especially if the fragment completely escapes the engine inlet.
Accordingly, there is a need for a fan case that allows the fan rotor to orbit during a fan blade-out event and causes blade fragments to exit the fan plane in the aft direction.
The above-mentioned need is met by the present invention which provides a fan case for a turbofan engine having a fan including plurality of fan blades mounted to a rotor disk and a decoupler that fails in response to a predetermined load. The fan case includes a substantially annular shell having a forward section, an intermediate section, and an aft section. The intermediate section being axially aligned with the fan and having a forward end. The fan case further includes an aft facing step formed at the forward end of the intermediate section.
The present invention and its advantages over the prior art will become apparent upon reading the following detailed description and the appended claims with reference to the accompanying drawings.